1. Field of the Invention
The present invention relates to an imaging device and a focusing control method.
2. Description of the Related Art
In recent years, according to the increase in resolution of an imaging element such as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor, demand for information devices having an imaging function, such as a digital still camera, a digital video camera, a mobile phone such as a smart phone, or a personal digital assistant (PDA) has rapidly increased. Such an information device having an imaging function is referred to as an imaging device.
In such an imaging device, as a focusing control method for focusing on a main subject, a phase difference auto-focus (AF) method (for example, see JP2011-135191A, JP2010-152161A, and JP2013-218137A) is employed.
In a solid-state imaging element mounted on an imaging device that performs a focusing control based on the phase difference AF method, a configuration in which pairs of phase difference detection pixels in which light shielding film openings are eccentric in opposite directions are discretely arranged on an entire imaging surface is used.
Such a phase difference detection pixel has a low sensitivity compared with that of a normal imaging pixel in which a light shielding film opening is not eccentric with respect to a photoelectric conversion unit. Thus, in a case where a subject has a low luminous intensity, a detection signal level of the phase difference detection pixel is lowered.
In order to compensate for the reduction of the detection signal level, if a gain-up process is merely performed to perform a correlation operation, an error occurs in a result of the correlation operation. Accordingly, a process of increasing a signal level by adding up detection signals of phase difference detection pixels is performed.
JP2011-135191A discloses a configuration in which detection signals of plural phase difference detection pixels which are diagonally arranged are added up and a correlation operation is performed using detection signals after addition to calculate a defocus amount.
Further, JP2010-152161A and JP2013-218137A disclose a configuration in which detection signals of plural phase difference detection pixels which are arranged at the same horizontal positions are added up and a defocus amount is calculated based on detection signals after addition.